The goal of this BIRT proposal is to build a new interdisciplinary team that will study the role of integrins in the regulation of chondrocyte mechanotransduction. An improved understanding of chondrocyte mechanotransduction under physiologic and pathologic loading conditions is needed in order to define the mechanisms by which biomechanical factors contribute to the development of osteoarthritis (OA). The focus of the parent project has been to determine the basic mechanisms that control cellular signals generated through the ?5?1 integrin in response to fibronectin fragments which stimulate production of catabolic mediators, including cytokines and matrix metalloproteinases (MMPs). We will build on these studies and take the project in a new direction by examining chondrocyte mechanotransduction pathways. Despite studies in other cell types implicating integrins as key mechanoreceptors, relatively little work has been done on their role in cartilage. The previous work in this area used cells in monolayer culture, which is quite different from the normal chondrocyte environment, and rarely examined adult human chondrocytes. The BIRT project will test the overall hypothesis that physiologic loading stimulates anabolic signaling mediated by the PI-3 kinase-Akt pathway while excessive (pathologic) loading stimulates catabolic signaling mediated by the MAP kinase (ERK, JNK, and/or p38) pathways. Given mounting evidence that Rho GTPase family members play pivotal roles in redox signaling and our recent work showing that Rac1 activity is required for fibronectin fragment induced MMP-13 expression, we will determine if redox regulation of Rho GTPases mediates the switch from anabolic to catabolic signaling in response to increased loads. In order to address this important knowledge gap, we propose to develop a new collaborative team that combines the expertise of the Loeser lab in chondrocyte integrin signaling, the expertise of Dr. Elizabeth Loboa's lab in biomechanics and the expertise of Dr. Keith Burridge's lab in integrin mediated mechanotransduction with a particular focus on Rho GTPases. The team will address the following aims: 1) Measure the activation of Akt and the MAP kinases and downstream anabolic and catabolic gene expression in response to physiologic and pathologic loading conditions in human chondrocytes. 2) Determine the role of the ?5?1 integrin and associated integrin signaling proteins in chondrocyte mechanotransduction in response to physiologic and pathologic loading. 3) Determine if Rho GTPases mediate redox signaling which regulates the switch from anabolic to catabolic gene expression as loading moves from physiologic to pathologic. In order to study chondrocytes in a 3-D environment, we will take the innovative approach of using cells cultured in suspension in microcarriers. Defining the components of the mechanotransduction signaling network that regulate anabolic and catabolic activity in cartilage will significantly advance the field by providing a more complete understanding of how biomechanical factors contribute to joint homeostasis and to OA.